During the first year of this award, we have demonstrated, using conventional microelectrode techniques, that there are significant differences in resting membrane and action potential characteristics of newborn and adult canine Parkinje fibers. Resting membrane potential of neonatal fibers were also shown to not be affected by ouabain (2 times ten to the minus seventh power M) while adult fibers depolarized significantly. Use of liquid potassium-exchanger microelectrodes to measure intracellular potassium ion activity (aik) indicate; (1) that differences in normal adult and neonatal Purkinje fiber resting potential can not be accounted for by differences in aik, and (b) that ouabain does significantly lower neonatal fiber aik (as it does in adult fibers) without a concomitant loss of resting potential.